The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for current matching. Merely by way of example, the invention has been applied to current matching of LED strings. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) have been widely used in various electronics products, such as LED/LCD TVs. Often, a LED/LCD TV includes multiple LED strings. FIG. 1 is a simplified conventional diagram showing a system for driving multiple strings of LEDs. Each of the LED strings 1021, . . . , 102n, where n is no less than 1, has one or more LEDs connected in series. The system 100 includes bipolar junction transistors 1041, . . . , 104n for driving the LED strings 1021, . . . , 102n respectively. In addition, the system 100 includes a controller 108. The emitter terminals 1101, . . . , 110n of the bipolar junction transistors (BJTs) 1041, . . . , 104n are connected to resistors 1061, . . . , 106n, respectively. In order to match currents 1121, . . . , 112n flowing through the LED strings 1021, . . . , 102n respectively, voltages at the emitter terminals 1101, . . . , 110n are often kept equal or approximately equal in magnitude. However, conventional techniques often cannot match the LED currents with satisfactory accuracy, for example, due to the parameter variations of bipolar junction transistors.
Hence it is highly desirable to improve techniques of current matching of LED strings.